LNF> Have two timer settings, one for battery usage, and one for external
LNF> power usage.
LNF> When on external power, the spindown timer is treated the same,
LNF> regardless of MP3 playback or user activity. It spins down after X
LNF> seconds of inactivity.

LNF> When on batteries, the HD spins down after X seconds inactivity after
LNF> user interaction. However, if the disk spins up due to MP3 data
LNF> loading (no user interaction) it will spin down again without delay.

That is what I'd also prefer.

LNF> The setting parameters would be "always on"..1..2..3..4..5..6.. and
LNF> so on.